Organic compounds can take various structures compared with inorganic compounds, and have possibility to provide materials having various functions by appropriate molecular design. Owing to these advantages, photo electronics and electronics which utilize functional organic materials have been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as an electronic device utilizing an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is considered that light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes which interpose a light emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the light emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to a ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible from any of these excited states.
Such a light-emitting element has a lot of problems which depend on the organic materials. In order to solve these problems, improvement of an element structure, development of a material, and the like have been carried out.
As the most basic structure of a light-emitting element, the following structure is known: a hole transporting layer, formed by an organic compound having a hole transporting property, and an electron transporting light emitting layer, formed by an organic compound having an electron transporting property, are stacked to form a thin film of approximately 100 nm thickness in total, and this thin film is interposed between electrodes (see Non-Patent Document 1, for example).
When a voltage is applied to the light-emitting element described in Non-Patent Document 1, light emission can be obtained from organic compounds having a light-emitting property and an electron transporting property.
Further, in the light-emitting element described in Non-Patent Document 1, functions are appropriately separated. That is, a hole transporting layer transports holes, and an electron transporting layer transports electrons and emits light. However, various interactions (for example, formation of exciplex, and the like) frequently occur at an interface of stacked layers. As a result, change of emission spectrum and/or decrease in luminous efficiency may take place.
In order to suppress change of emission spectrum and decrease in luminous efficiency which are caused by the interaction at an interface, a light-emitting element was developed in which functions are further separated. For example, a light-emitting element has been proposed, in which a light emitting layer is interposed between a hole transporting layer and an electron transporting layer (see Non-Patent Document 2, for example).
In such a light-emitting element described in Non-Patent Document 2, it is preferred that, in order to more effectively suppress the interaction occurring at an interface, a light emitting layer is fabricated by using a bipolar organic compound which has both an electron transporting property and a hole transporting property.
However, most organic compounds are monopolar materials having either a hole transporting property or an electron transporting property.
Therefore, a bipolar organic compound having both an electron transporting property and a hole transporting property has been required to be developed.
In Patent Document 1, a bipolar quinoxaline derivative is described. However, since their performances such as thermal stability are not satisfactory, bipolar organic compounds having high thermal stability have been required to be developed.
[Non-Patent Document 1]
    C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)[Non-Patent Document 2]    Chihaya Adachi et al., Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)[Patent Document 1]    PCT International Publication No. 2004/094389